The present invention relates to a low temperature polysilicon film and in particular to a polysilicon structure formed by crystallization of an amorphous silicon structure, methods for forming the same, and devices utilizing the film.
Typically, thin film transistors (TFTs) are used as the active devices in active matrix flat panel displays. For example, TFTs are employed to drive the liquid crystal display (LCD) or the organic light emitting display (OLED).
Conventionally, hydrogenated amorphous silicon (α-Si: H) is used as the semiconductor film (active layer) of a TFT. Polysilicon, however, may provide higher electron transmission than amorphous silicon due to more regular crystal orientation. Thus a development trend is to utilize the polysilicon instead of amorphous silicon in TFT technology.
Typically, there are three methods for forming the thin polysilicon film. In a first method, the polysilicon film is formed by deposition. In this method, the polysilicon film requires a sufficient thickness, so as to grow large grains. Thus, the polysilicon film has a poor surface uniformity that adversely affects the formation of the subsequent gate insulating layer. Moreover, the deposition temperature is high (600° C.), which also adversely affects the fabrication of the device. In a second method, thermal treatment is performed on an amorphous silicon layer, so as to transfer to a polysilicon layer. In this method, the polysilicon layer may be less thick and have a better surface uniformity. However, a high deposition temperature (600° C.) and long deposition duration are required and the thermal budget increases. As a result, throughput and device reliability suffer. In a third method, perform a laser treatment to transfer an amorphous silicon layer into a polysilicon layer. The method is the most commonly used.
The polysilicon TFT display comprises a display region and a driving circuit region, wherein the switching devices on the driving circuit region require higher switching rate and readability. That is, the switching devices on the driving circuit region preferably have higher electron transmission and better sub-threshold swing. A polysilicon layer with large grain size can provide such electronic characteristics. Additionally, current leakage in the display region must be low. If the surface roughness of the polysilicon layer is too high, poor coverage of the gate insulating layer thereon results, thus increasing current leakage. If the grain size of the polysilicon layer is small, low surface roughness results, improving the coverage of the gate insulating layer thereon and reducing current leakage. That is, in order to improve the electronic characteristics of the polysilicon TFT display, the polysilicon layer on the driving circuit region must have a relatively large grain size and that on the display region a relatively small grain size.
In order to form a polysilicon film with different grain sizes on the driving circuit region and the display region, respectively, for fabrication of the polysilicon TFT display, the driving circuit region and the display region must be respectively treated. For example, a laser treatment with a relatively low scanning rate is performed on the amorphous silicon layer on the driving circuit region, to form the polysilicon layer with a relatively large grain size. The laser treatment with a relatively high scanning rate is subsequently performed on the amorphous silicon layer on the display region, to form the polysilicon layer with relatively small grain size. The problems presented by laser alignment and mask changes may reduce yield and throughput. Thus, an improved method for forming polysilicon layers with different grain sizes is desirable.